The invention disclosed herein relates to a process and apparatus for measuring the load bearing capacity of traffic-carrying surfaces such as roadways, runways, piers, terminals, and the like.
The need to measure the load bearing capacity of traffic-carrying surfaces arises in a variety of contexts. For example, there is a need to determine the load bearing capacity of newly constructed surfaces to determine whether the meet stated specifications. In addition, it is useful to be able to measure the load bearing capacity of a surface to determine whether reinforcement is required, as well as the extent to which the surface might be damaged when deciding whether to allow an especially heavy load to be transported over the surface. Also, the load bearing capacity of certain surfaces change with the weather and the seasons so that it is useful to be able to determine and post the maximum load capacity of the surface at various times and under various conditions. The expected life of a particular traffic carrying surface can also be estimated based on the maximum loads to be allowed thereon and the estimated frequency with which the surface will be subjected to such loads. It is also of interest to be able to quickly, accurately, and inexpensively discover weaknesses in a traffic carrying surface at an early stage so that they can be repaired promptly, and major construction, with attendant higher costs, can be avoided.
It is known that an estimate of the load bearing capacity of a surface can be obtained by examining the deformation of the surface when a known load is applied. The form and degree of the resulting depression or deformation are influenced by inhomogeneities in the surface material, vertical and lateral variations in the packing of the underlying layers, moisture conditions, proximity to the surface edge, and other factors which effect the maximum safe load bearing capacity. Generally, the maximum safe load bearing capacity varies inversely with the degree of surface deformation observed. Using modern mechanical measuring instruments, which can accurately measure surface deformation in the order of 0.1-0.2 millimeters, it has been found, for example, that even the very best roadways will exhibit deformation in the range of 0.1 millimeters near the point where the load contacts the surface. It is also known that the load bearing capacity of a particular surface is evidenced not only by the longitudinal deformation of the surface when under load but also by the lateral deformation thereof. For example, whereas a longitudinal "funnel" deformation is indicative of deep underlying damage, a broad, shallow, transverse deformation indicates shallower superficial damage.
A proliferation of new highways, airports, shipping terminals, and the like, and a marked increase in the volume and weight of the traffic using these facilities have been observed over the past several decades. As a result, the ability to quickly, accurately, and efficiently measure the load bearing capacities of such surfaces with minimal interference in each of the contexts previously mentioned has become of significant importance. Previously, a number of attempts have been made, based on the measurement of surface deformation with a known load, to satisfy the need. However, for a variety of reasons, each of these attempts has produced less than satisfactory results.
One approach includes dropping on angularlyshaped pointed weight onto the surface being tested and determining the load bearing capacity from the resulting deformation of the road surface. This approach has been found unsatisfactory in that it is time consuming and requires the interruption of traffic flow on the portion of surface being tested.
Another approach employs a slowly or intermittently advancing vehicle to carry a "stamping" device which periodically "stamps" the surface, after which the deformation thereof is measured. Again, this approach is time consuming, and in addition creates a traffic hazard due to the slow speed of the testing vehicle.
Still another approach involves exposing the surface being tested to vibrations on a load from a stationary testing vehicle, and then measuring the depth and shape of the resulting depression. This approach likewise is time consuming. In addition, it can cause a certain amount of damage to the surface and is expensive.
An additional problem common to approaches wherein surface deformation is measured in response to the application of a stationary or slowly moving load is that the measured deformation does not accurately reflect the true load bearing capacity of the surface at the load velocities to which it is likely to be subjected. Such approaches generally underestimate the true load bearing capacity of a surface because a stationary or slowly moving load produces a greater and longer lasting deformation of the surface than does the same load travelling at normal highway velocity, for example.
An overview of these and other prior art approaches to measuring the load bearing capacity of traffic-carrying surfaces is contained in the following documents: Nielsen, Sammenliguende Beareernemalinger med Forskelligt Inventeringsudestyr ("Comparative Carrying Capacity With Various Data Gathering Equipment"), State Highways Laboratory, Report No. 124, Roskilde (July 1981); Reinslett, Carrying Capacity of Roads, Directorate of Roads, Road Laboratory, Oslo, Norway (1982); and Lampinen, Variation of the Carrying Capacity in Transverse Profile of the Road, State Technical Research Center, Research Report No. 122, Esbo (1983).
In contrast to previous approaches, measuring the load bearing capacity of traffic-carrying surfaces according to the present invention has the objects and advantages of not causing damage to the surfaces, not interfering with the normal flow of traffic, generating a large number and density of measurements faster and more efficiently than previous approaches, and producing results that much more accurately reflect the true load bearing capacity of the surfaces measured. Further, one presently preferred embodiment of the present invention has the additional objects and advantages of measuring transverse elevational profiles to provide measuring-vehicle-independent surface depression values, and of obtaining data concerning lateral surface deformation to increase the accuracy of the determination of the load bearing capacity of surfaces.